Article contributed by Mr. A. K. Singh, Assistant Professor, Faculty of Technology and Computer Applications
One of the most important system of an automobile is brakes .They are required to prevent the vehicle within the smallest possible distance and is it is done by converting kinetic energy of the vehicle into heat which is dissipated into atmosphere.The main requirements of brakes are given below:-
1. The brakes must be strong enough to stop the vehicle within the minimum possible distance in an emergency. But this should also be consistent with safety. The driver must have a correct control over the vehicle during emergency braking and therefore the vehicle must not skid.
2. The brakes must have good antifade characteristics and its effectiveness should not decrease with constant prolonged application.
The actual stopping distance of vehicle when braking is done is Vehicle speed, Condition of the paved surface , Condition of tyre tread, Coefficient of friction between the tyre tread and the road surface, Coefficient of friction between the brake disc and brake friction pad, Braking force applied by the driving force .
A disk brake consists of a forged iron disc bolted to the wheel hub and a stationary housing called caliper. The caliper is connected to some stationary part of the vehicle, just like the axle casing or the stub axle and is cast in two parts, each part containing a piston. In between each piston and disc there’s a friction pad held in position by retaining pins, spring plates etc., passages are drilled within the caliper for the fluid to enter or leave each housing. These passages also are connected to a different one for bleeding.
When the brakes are applied hydraulically actuated pistons move the friction pads into contact with the disc, applying equal and opposite forces the later. On releasing the brakes the rubber sealing rings act as return springs and retract the pistons and therefore the friction pads far away from the disc.
Two types of brake discs are generally used:
- Solid type
- Ventilated type.
The ventilated type brake disc is more efficient since it provides better cooling. But the disadvantage is that they are thicker and heavier than solid type, they are susceptible to wrap at severe brakingconditions, the dirt accumulates within the vents which affects cooling and apart produces wheel imbalance.
The discs of the brakes are made from pearlite gray forged iron. The material is reasonable and has good antiwear properties. The other materials used for the manufacture of disc are Aluminium and Ceramic.
Obviously the cast-iron disc is that the heaviest a part of a brake near about 8 kg/per wheel on car. Aluminium alloy discs are utilized due to light in weight, they are less immune to heat and fade,
In contrast, carbon-fiber disc is most heat-resisting yet is far and away the lightest, however, it requires very high working temperature, and otherwise braking power and response are going to be unacceptable.
Ceramics are inorganic, non-metallic materials that are processed and used at high temperatures. They are generally hard brittle materials that withstand compression alright but don’t delay well under tension compared to the metals. They are abrasive-resistant, heat resistant (refractory) and may sustain large compressive loads even at high temperatures. The different sorts of ceramics are clays, refractories, glasses etc.
COMMPARISON OF CERAMIC BRAKES AND CONVENTIONAL BRAKE DISC
Till now brake discs are made from grey forged iron, but these are heavy which reduces acceleration, uses more fuel and features a high gyroscopic effect.
Ceramic disk brake weight less than carbon discs but have an equivalent frictional values. Carbon discs are used only in Formula 1 racing cars etc., because it’s so expensive. More over ceramic brake discs are good even in wet conditions which carbon disc notoriously fails to do so.
But comparing their weight, we will see directly that we are watching two different worlds, with ceramic brake discs quite 61 per cent lighter than conventional forged iron discs. In practice this reduces the load of the car, counting on the dimensions of the brake discs, by up to twenty kg. And aside from saving fuel, leading to better and lower emission for an equivalent mileage, this also means a reduction in masses with an extra improvement of shock absorber response and behavior. Another is that the manufacturer can add more safety features without adding to current weight.
MANUFACTURING OF CERAMIC BRAKE DISCS
The composites for producing fiber reinforced ceramic brake discs are short carbon fibers, carbon powder, and resin mix. This method involves first compressing the carbon fibers, carbon powder and therefore the resin mix together then sintering at 1000°C, within the furnace a stable carbon frame work created. This consists of carbon fibers during a carbon matrix. Once cooled this material areoften ground like wood and therefore the break disk obtains its final shape.
Together with silicon the bottom break disk blank is then inserted into the furnace a second time. The pores within the carbon framework absorb the silicon melt sort of a sponge; the fibers themselves remain unaffected by this process.
Thermosetting resins are those resins which, during molding process (by heating) get hardened and once they need solidified, they can’t be softened i.e. they’re permanent setting resins. Such resins during moldings, acquire three dimensional cross linked structure with predominantly strong covalent bonds. They formed by condensation polymerization and are stronger and harder than thermoplastic resins. They are hard, rigid, and waterproof andscratch resistant.
ADVANTAGES OF CERAMIC BRAKE DISCS
1. Ceramic brake discs are 50% lighter than metal brake discs. As a result, they will reduce the load of car by up to 20kg. Just in case of a high speed I.C.Engine like train with 36 brake discs, these savings amount to six tons. And aside from saving fuel, this also means a discount in unsprung masses with an extra improvement of shock response and behavior.
2. The ceramic brake disc ensures very high and, especially, consistent frictional valuesthroughout the whole deceleration process. With Porsche ceramic brake discs, a car was ready to decelerate from 100Km to 0Km in but 3 seconds.
3. Brake temperature – an element crucial to stopping distances with metal brake discs – is now only a minor factor, both the lining and ceramic brake disc retaining their high level of friction no matter whether or not they are hot or cold.
Ceramics retain their resistance up to 2000°.Only if the temperature is more than this, they loose their
5. Initial field studies have shown that ceramic brake discs can still reliably bring an automobile tostandstill even after 300,000 kilometers.
6. They are not subjected to wear, maintenance free and are heat and rust resistant.
7. Heavily commercial vehicles are often braked safely over long distances without having to undergo brake maintenance. This dispenses with the necessity for expensive maintenance.
8. Ceramic brake discs don’t rust under high oxygen concentration.
9. Dry and wet performances are excellent. Ceramics are water proof materials and therefore the restraint always remain dry.
The main disadvantage of ceramic brake discs is their high initial cost. But, because of the advantages listed above, the ceramic brakes will work out to be cheaper in the long run.
Ceramic brake discs due to its advantages over the conventional brake discs are going to be the brake discs for cars in future. With the success of this in Porsche turbo car, many other racing cars and commercial vehicles are going to implement the ceramic disc in their cars.